Satellite radio is an emerging technology that is in the early stages of gaining consumer acceptance. Major electronics manufacturers such as Pioneer, Alpine, Carion, Delphi Delco, Sony, and Motorola and automobile companies, such as General Motors and Honda, are partnering with satellite radio providers to bring satellite radio to the consumer. Satellite radio enables users to subscribe to a service by which high quality audio content, free of the interference often accompanying traditional radio frequency (RF) broadcast systems, is available via satellite transmission. In addition, in vehicular use, satellite radio enables a vehicle equipped with the appropriate receiving equipment to hear the same station regardless of the vehicle location, i.e., a vehicle could travel from New York to Los Angeles without losing the signal of a particular station to which the receiving equipment is tuned.
Typically, a satellite radio service provider utilizes at least two satellites, although a single satellite or more than two satellites may be used, depending upon the amount of coverage area desired. A ground station transmits a signal containing the content to the two satellites, which reflect the signals back to earth where they can be received by radio receivers possessed by subscribers. The radio receivers are programmed to receive the signals and unscramble them so that the content of the transmission can be enjoyed by the listener. Various other information can be included in the broadcast signal, for example, information regarding the artist and title of a particular song being played, which can then be displayed on the receiver unit.
A major obstacle to achieving efficient and reliable data communications over traditional radio frequency channels in urban areas is multipath propagation, also called multipath fading. Multipath fading is the degradation, i.e., fading, of a radio signal that occurs when multiple copies of the same radio signal arrive at the receiver through different reflected paths. This can happen when, for example, there are signals reflected off of buildings, trees, other vehicles, etc. that arrive at the receiver at the same or different times. The interference of these signals, each having traveled a different distance, results in phase and amplitude constructive and/or destructive addition. This can result in severe and rapid fluctuations in the received signal strength, particularly with a mobile receiver as it is moved. The duration of a fade depends on the velocity of the receiver and is typically on the order of a few milliseconds.
The problem of multipath fading can be somewhat reduced through the use of repeaters. Typical satellite radio systems utilize one or more ground-based or roof-top-based repeaters in urban areas. The repeaters receive the signal from the satellites and re-transmit it on a more local basis to receivers in the urban area. While this decreases the problems caused by deflected signals, it does not eliminate the problem altogether and thus multipath fading and its associated problems persists.
In a conventional serial modulation scheme (also known as “single carrier” or “single channel” scheme), data bits are transmitted over a single channel sequentially. If a deep fade (a fading signal where the signal-to-noise ratio is very low) occurs during the transmission of such a signal, then the bits that are transmitted during the deep fade cannot be received correctly. To solve this problem, a technique known as “dithering” has been used, whereby the single carrier is transmitted using multiple antennas, and a different fixed-frequency phase offset is applied to each of the transmitted signals. By using this technique, each transmitted signal is effectively a different channel, each with independent multipath fading characteristics. In this way, the receiver receives multiple independent signals, not all of which are experiencing a deep fade, and performance is improved. Examples of dithering can be found in U.S. Pat. No. 6,157,612 to Weerackody et al; U.S. Pat. No. 5,289,499 to Weerackody; and U.S. Pat. No. 5,577,265 to Wheatley, III, each of which are incorporated fully herein by reference. The developments disclosed in each of these patents require the use of fixed-frequency phase offsets to provide the multiple independent copies of the transmitted signal.
Another known way to deal with multipath fading is to transmit a data frame containing a block of bits in parallel over a multichannel path at a low baud rate so that the time taken to transmit the frame is relatively long (typically, for example, on the order of a fraction of a second) relative to the expected duration of a fade. The effect of a fade is then spread out over many bits. Rather than a few adjacent bits being completely destroyed by a fade, all of the bits in the frame are slightly affected by a fade which occurs during the time that the frame is being transmitted.
One well-known and effective scheme for transmitting a block of bits in parallel over a channel is called orthogonal frequency-division multiplexing, or OFDM. OFDM is a wireless technology that operates on the principle of transmitting data by dividing the data stream into multiple, parallel bit streams that have a much lower bit rate and using these multiple bit-streams to modulate several low bandwidth sub-carriers or sub-channels. These sub-channels are orthogonal, which means every sub-channel can be separated out of the receiver without interference from the other sub-channels. This is made possible due to the mathematical property of orthogonal wave forms, which ensures that the integral of the product of any two sub-channels is zero. Therefore, by dividing up the frequency band into a large number of narrow-band sub-channels, wireless channel impairments, such as multipath fading, are significantly reduced. Fading impacts a very limited number of the sub-channels, and most of the narrow-band sub-channels, along with the information modulated into the wave form, are communicated over the channel in a reliable manner. Therefore, OFDM provides for superior link quality and robustness of communication over the wireless channel.
As mentioned above, OFDM is basically the dividing up of a single-channel transmission into multiple narrow-band sub-channels, prior to transmitting the group of sub-channels together over a single channel as a complete signal. When conditions are such that one or more of the sub-channels experiences a severe fading problem, problems still occur in the overall channel (the complete signal) when the sub-channels experiencing the fade contain information critical to the transmission, for example, synchronization information. When a vehicle with a satellite receiver experiences such a fade and is moving slowly or is stationary (i.e., so that it remains in a location subject to the fade for an extended time period), the information contained in this missing channel may disable the receiver's ability to process the incoming signal altogether. Accordingly, it would be desirable to have a method and system whereby the occurrence of deep fades in the sub-channels of an OFDM system can be minimized.